The present invention relates to telecommunications networks. More specifically, it relates to echo cancellation devices used in two-wire line full duplex communication systems, and more particularly to two-wire line modem devices.
Past implementations of hybrids used to accomplish four-wire to two-wire conversion employ a transmission signal cancellation path to duplicate and cancel the portion of the outgoing transmission signal that is erroneously picked up by the receive amplifier. A hybrid used in a typical solid-state Data Access Arrangement (DAA) device will employ a gain stage that serves as the transmission signal cancellation path.
Telephone lines present a wide range of characteristic impedances. The characteristic impedance varies with frequency on a particular telephone line, and each two-wire subscriber line, or telephone line, will present a different characteristic.
Hybrids in present DAA devices use a fixed gain stage for cancellation that is not responsive to changes in the telephone line impedance. Such a fixed gain stage does not provide frequency dependent impedance matching. The DAA is designed to perform a coarse echo cancellation by providing a hybrid that is not precisely balanced. The prior art DAA devices do not provide for the connection of external impedance matching devices.
In view of recent telecommunication techniques that utilize a larger portion of the available bandwidth such as that specified by ITU-T recommendation V.90, it would be advantageous to provide a transmission signal cancellation path in a DAA device that is more flexible and responsive to varying telephone line conditions, and which is able to better model the telephone line impedance to provide better echo cancellation, especially at the band edges.
In accordance with preferred embodiments of the present invention, an improved data access arrangement is provided. The DAA provides an external pin for connecting an impedance matching circuit, and includes a variable gain stage to accommodate a variety of line impedance characterisitics. An improved data access arrangement device for use in full-duplex two-wire communication systems having a transmission impedance characteristic. The DAA includes a transmit amplifier and a receive amplifier adapted for connection to a two-wire line, a codec connected to the transmit amplifier for generating a transmit signal, a summation device connected to the receive amplifier, an echo cancellation path including a variable gain amplifier, where the path extends between the codec and the summation device, and wherein the variable gain amplifier is adjusted in accordance with the transmission impedance characteristic. The variable gain amplifier may include a gain register that is programmed by a microprocessor, or may have the gain set by user-configurable switches. The echo cancellation path also may include a frequency dependent impedance circuit, and more specifically, a bandpass filter. The impedance circuit may also be an active filter element, such as an operational amplifier. The filter is preferably external to the DAA integrated circuit so as to allow flexibility in the choice of filter parameters. The summation device is preferably an operational amplifier.